An apparatus such as an optical disk apparatus or a magnetic disk apparatus is generally arranged (1) to perform tracking servo control for positioning, on a specific track of a disk, an optical pickup or a magnetic head so as to record and reproduce information to and from the disk, and (2) to perform access control for quickly moving the optical pickup or the magnetic head to a target track. Therefore, such an apparatus requires a motor for moving the optical head or pickup in a radial direction of the disk.
Problems, arising from the motor, such as distortion in characteristics including occurrence of mechanical resonance characteristics, changes of sensitivity (parameters), or disturbances due to vibrations and shocks, adversely affect control characteristics of the motor. To counteract these problems, some studies have been made.
For example, a study on control of joints of a robot, which intends to cope with the above-mentioned problems, is reported by Kohei Ohnishi (the Journal of the Institute of Electrical Engineers of Japan, Vol.110, No. 8, 1990, pp. 657-660).
According to the article, it is considered that deviation from the ideal characteristics and response of a motor due to characteristic distortion, parameter variations, vibrations or shocks is attributed solely to a disturbance torque applied to the motor. The article discloses a method utilizing a so-called disturbance observer, by which the disturbance torques are collectively estimated and fed back to the motor.
FIG. 36 is a block diagram illustrating the system utilizing the disturbance observer, and FIG. 37 is a block diagram illustrating the system which is equivalently re-drawn.
In FIG. 37, the right half configuration of a disturbance observer 513 is for obtaining entire torque T.sub.1, which is applied to the motor 510, while the left half configuration is for obtaining a nominal value T.sub.2 of driving torque T.sub.m. The torque T.sub.1, includes the driving torque T.sub.m and disturbance torque T.sub.dis, and is obtained by (1) conducting approximate differential operation so as to convert a rotation velocity (angular velocity) .omega. of the motor 510, which is detected by a sensor, into an angular acceleration in the bandwidth whose upper angular frequency is g, and then (2) multiplying the angular acceleration by the amount of reduced gain g during the approximate differential operation and a nominal value J.sub.n of the moment of inertia J. The nominal value T.sub.2 of the driving torque T.sub.m is obtained by multiplying drive current I of the motor 510 by a nominal value K.sub..tau.n of torque constant K.sub..tau..
The disturbance observer 513 obtains an estimated value PT.sub.dis of the disturbance torque T.sub.dis in accordance with the difference (T.sub.2 -T.sub.1) and multiplies this difference by a reciprocal of the nominal value K.sub..tau. n of the torque constant K.sub..tau. so as to obtain a compensating current I.sub.cmp for canceling the above-described difference.
According to the system using the disturbance observer 513, the compensating current I.sub.cmp is added to a reference drive current I.sub.a.sup.ref for the motor 510 so as to obtain an actual drive current I, and the actual drive current I is supplied to the motor 510 so as to suppress the affections in the motor 510 due to such as the various characteristic distortions, the parameter variations, the vibrations and shocks applied from outside. As a result, the transfer function from the reference drive current I.sub.a.sup.ref to rotation angle .theta. is given as K.sub..tau.n /J.sub.n .multidot.s.sup.2 (s: Laplace operator), which is determined by the nominal values of the torque constant K.sub..tau. and the moment of inertia J.
FIGS. 36 and 37 indicate that the motor has a constant term D of viscosity. However, the disturbance observer 513 does not have a block which specifies it. Therefore, it is contained in the disturbance torque T.sub.dis, and the disturbance torque T.sub.dis is suppressed, thereby never appearing.
The low-pass filter {g/(s+g)} in the left half configuration of the disturbance observer 513 is derived from the equivalent conversion. The low-pass filter has no affection at angular frequencies not higher than the angular frequency g, in which a signal is approximate-differentiated, while it affects on the bandwidth at angular frequencies higher than g.
The use of such a disturbance observer for controlling a motor, however, causes the following problem. When the motor or other elements driven by the motor are moved beyond the motion space and come into collisions, or when great shocks are applied to these elements, it is likely that the motor is greatly driven, thereby damaging the motor and the elements driven by the motor.
For example, it is assumed, in a machine with a structure such that an arm is rotated by a motor, that when the rotating arm is abruptly obstructed by, for example, collisions of the arm against a wall, this means for the arm and the motor that a disturbance acceleration (or a disturbance angular acceleration) is abruptly applied to them and obstruct their normal action. Accordingly, the disturbance observer further drives the motor so as to offset it, thereby causing the arm to be further forcibly pushed against the wall.
As a result, mechanical or electrical overload is given to the arm and the motor, and they are damaged in the worst case. This is not restricted to such collisions against a wall. Even when some great shocks happen to the motor or the arm, the disturbance observer also regards it as the shocks obstructing the normal motion of the motor, thereby causing the same phenomenon.
These problems are not unique to such a driving system utilizing a disturbance observer for driving a rotary motor, but are common to a driving system for driving a linear motor often utilized in an optical disk apparatus, as well as a swing motor for use in a magnetic disk apparatus. In addition, in an apparatus for recording and reproducing information, such as the optical disk apparatus or the magnetic disk apparatus, the motor moves minute and damageable elements such as the optical pickup or the magnetic head. Therefore, the affection of such a problem is necessarily great. Accordingly, it has been difficult to apply the disturbance observer to such an apparatus so as to control a motor, in spite of its preferable effects such as suppressing characteristic distortion of the motor and disturbances.